Hi Sam,
The DVR systems that I have seen (only a few of them, I must admit) have had an option to run a web server and allow remote users to monitor from anywhere in the world. These have allowed login through either a webpage or via an app on a smartphone, like you say. I have connected a few to iPads (as an example). It is cool technology and can be extremely useful.
As you mention, the defenses preventing an attacker from hacking in to these CCTV/DVR systems and viewing your cameras/recordings come down to two:
- obscurity (who would bother to come looking)? and
- strong username and password (can just anyone login)?
The best defence would be to not advertize the make and model of the DVR system, or that there is a web-viewing option at all. Ensure that there are very strong usernames and passwords configured, and check the logs to see if any connection attempts are being made. If it is possible, configure the system to lock out failed logins after a few failed attempts.
And, of course, if this web option is never being used... don't run that webserver service! &: )
Hope this helps!